It is known to create interior (or female) threads with the aid of a screw tap or a thread forming tap. Screw taps evolve from screw bolts by generating cutting edges through the introduction of axis parallel or slant flutes. In the lead-in region, a so-called starting taper is provided, which is mostly realised in conical form. The hole in the workpiece is pilot-drilled with the core diameter.
Thread forming taps are also geometrically derived from a screw bolt. However, they do not remove any material, but only displace it. The workpiece is therefore pilot-drilled with the average pitch diameter. The thread forming tap has an ascending profile on the cone point, which progressively displaces the material.
It is known to increase the wear resistance of thread forming taps through selection of the material they are made from and by heat treatment. On this occasion, particular advantages are brought about by coating with titanium nitride (Maschinenmarkt, Würzburg 92 (1986) 40, pages 88 to 92). But it is also known to form thread forming taps from hard metal (Maschine+Werkzeug 12/95, pages 26 to 28).
From DE 39 34 621 C2, a thread forming tap has become known, with a cylindrical chuck shaft and a form-giving region with polygonal cross-section differing from the annular shape, as well as with thread corrugations on the outer perimeter which correspond to the thread which is to be produced. In the region of the cross section corners of the form-giving region, cylindrical ridges from particularly hard and wear resistant material are soldered into corresponding, axially running grooves, which ridges have a contour on the outer perimeter matching the thread corrugations. Thread forming taps have the same thread profile as screw taps, but have a polygonal cross section in their profile portion, with three or six “corners”, e.g.
The profile portion of thread forming taps and screw taps has a first forming region by which the thread is essentially produced, as well as a guiding portion following thereafter. This serves for the guiding of the tool when it is turned back from the bore, after completion of the interior thread.
Thread forming taps and screw taps are relatively expensive to manufacture and consist of a high-quality material, as the case may be. After wear or damage, the tool is no more useful.
The invention has the objective to create a cold thread forming tap or screw tap which can be produced with little expenditure of material and manufacture.